The determination of received signal impairment plays an important role in communication signal processing. For example, some types of interference-canceling receivers exploit the correlation of signal impairments between multipath components of the received signal to improve interference suppression. Generalized RAKE (G-RAKE) receivers exemplify such operations by generating a combined signal for demodulation based on combining multipath delay components of a received signal of interest using combining weights W that incorporate impairment correlation estimates.
In more detail, the combining weights W may be expressed as W=R−1h, where R−1 is the inverse of an impairment covariance matrix R, and h is the channel response vector. (The covariance matrix may be used to represent zero-mean impairment correlations.) G-RAKE combining thus depends on the calculation of the impairment correlation estimation, and similar dependencies exist in other types of interference-canceling receivers, such as chip equalizer architectures that calculate (equalization filter) weights W based on impairment correlations.
Further, received signal quality, ρ, may be expressed as a function of the weights (ρ=h*W=h*R−1h). Signal quality estimation, such as channel quality estimation, plays an important role in many types of wireless communication systems. For example, some systems use rate-controlled channels that transmit data to individual users at the highest rates permitted by the available transmit power and the prevailing user-specific radio conditions. The data rate selected for a given user depends on channel quality feedback from that user. The High Speed Downlink Packet Access channels in the Wideband Code Division Multiple Access (W-CDMA) standards represent one type of rate-controlled channel dependent on channel quality feedback, while the shared Forward Packet Data Channels (F-PDCHs) in the cdma2000 standards represent another type of rate-controlled channel.
Regardless of the particular standards involved, under-reporting channel quality generally results in system inefficiencies, because individual users are served at rates lower than could be supported by the actual conditions. Over-reporting channel qualities also leads to inefficiencies and, in fact, may be worse than under-reporting because the ARQ protocols often used in such systems generate excessive data retransmissions when data rates are set too high for the actual conditions.
With HSDPA signals, and similar types of signals in other communication network types, a number of users share a packet data channel in time-multiplexed fashion. For example, the information streams for multiple users may be time-multiplexed by a base station scheduler onto a shared packet data channel, such that only one user is being served at any given time. User-specific radio conditions and the currently available transmit power and spreading code resources at the transmitting base station determine the per-user data rates achieved on the shared channel.
Service schedulers oftentimes based ongoing scheduling decisions as a function of the data rates each user can be served at—i.e., schedulers often favor users in better radio conditions, since such users can be served at higher rates, which increases the aggregate data throughput of the shared channel. Therefore, individual users feed back channel quality estimates for the shared channel signal, on an ongoing basis, in support of dynamic scheduling. Practically, this fact means that users estimate channel quality for the shared channel during times whether or not they are actually receiving data on the share channel.
Accurate channel quality reporting in the above context is challenging in Single-Input-Single-Output (SISO) systems, and even more so in Multiple-Input-Multiple-Output (MIMO) and Multiple-Input-Single-Output (MISO) systems. Indeed, in systems having multiple transmit antennas, such as MIMO and MISO systems, data signals may be transmitted from more than one antenna, and data signal spreading codes may be reused across the antennas, i.e., multi-coding may be employed. Further, other signals—e.g., voice, dedicated packet, broadcast, control, and overhead channel signals—may be transmitted from one or more of the antennas.